Bottom Line:
In the GW3965 treated 3xTg-AD mice we also observed reduction in astrogliosis and increased number of stem and proliferating cells in the subgranular zone of the dentate gyrus.The effect of GW3965 on synaptic function was protein synthesis dependent.Our findings identify alternative functional/molecular mechanisms by which LXR agonists may exert their potential benefits as a therapeutic strategy against AD.

ABSTRACTAlzheimer's disease (AD) is the major cause of dementia worldwide. The pharmacological activation of nuclear receptors (Liver X receptors: LXRs or Retinoid X receptors: RXR) has been shown to induce overexpression of the ATP-Binding Cassette A1 (ABCA1) and Apolipoprotein E (ApoE), changes that are associated with improvement in cognition and reduction of amyloid beta pathology in amyloidogenic AD mouse models (i.e. APP, PS1: 2tg-AD). Here we investigated whether treatment with a specific LXR agonist has a measurable impact on the cognitive impairment in an amyloid and Tau AD mouse model (3xTg-AD: 12-months-old; three months treatment). The data suggests that the LXR agonist GW3965 is associated with increased expression of ApoE and ABCA1 in the hippocampus and cerebral cortex without a detectable reduction of the amyloid load. We also report that most cells overexpressing ApoE (86±12%) are neurons localized in the granular cell layer of the hippocampus and entorhinal cortex. In the GW3965 treated 3xTg-AD mice we also observed reduction in astrogliosis and increased number of stem and proliferating cells in the subgranular zone of the dentate gyrus. Additionally, we show that GW3965 rescued hippocampus long term synaptic plasticity, which had been disrupted by oligomeric amyloid beta peptides. The effect of GW3965 on synaptic function was protein synthesis dependent. Our findings identify alternative functional/molecular mechanisms by which LXR agonists may exert their potential benefits as a therapeutic strategy against AD.

Mentions:
At the functional level, AD has been proposed to be characterized by early synaptic abnormalities [37]. AD mouse models display changes in synaptic structure, function and plasticity at a young age, [38,39]. It has also been hypothesized that synaptic dysfunction is a major cause of the cognitive deficits observed in AD [27,29,40]. Because GW3965 improved cognitions in an hippocampus-dependent paradigm such as MWM, we tested whether the drug had any effect on hippocampus synaptic transmission. Acute application of 0.5 μM GW3965, produced a gradual potentiation of single fEPSPs evoked from Schaeffer collateral to CA1 neurons every 30 sec (Fig 9A), which was prevented by the protein synthesis inhibitors anisomycin or cyclohexemide [41] (Fig 9B and 9C). Interestingly 0.1 μM GW3965 did not significantly affected fEPSP amplitude or slope (Fig 9B and 9C), while 1 μM was synaptotoxic (not shown). As it has been reported before [42] also in a dose dependent manner, bath application of oligomeric Aβ42 (oAβ42) peptides at 200 nM concentration in hippocampal brain slices of WT mice inhibited the induction of long term potentiation of the Schaeffer collateral-CA1 synapses, a proposed electrophysiological correlate to learning and memory (Fig 9D). No significant effect on LTP induction or maintenance was observed with 100 nM (oAβ42) peptides or scrambled control Aβ peptides (Fig 9D). Since for the LTP paradigm it is necessary to obtain a baseline fEPSP for 15 min (see methods for details) we used 0.1 μM GW3965 co-incubation with oAβ42, as shown in (Fig 9E and 9G), GW3965 at this concentration rescued LTP function. These experiments suggest that GW3965 at 0.1 μM independently of the acute effect on fEPSP amplitude or slope can rescue LTP. We also rule out that the slice incubation with 200 nM oAβ42 or 0.1 μM GW3965+ 200 nM oAβ42 had any effect on basal fEPSP amplitude or slope for the time of the recording (S4 File). We then asked whether the effect of GW3965 on LTP modulation by oAβ42 required protein synthesis. Since LTP induction has been shown to be protein synthesis dependent [43,44], anisomycin was used to block protein synthesis 30 min after LTP was induced, as shown in (Fig 9F). The effect of GW was not affected by this treatment. These experiments are consistent with the observations made by other groups, that LTP induction is protein synthesis dependent, but not its maintenance (for the time we studied here). The experiments also suggest that once the effect of GW3965 is established (during the incubation period) then it is independent of protein synthesis. This does not imply that protein synthesis is not required during its initial effect. [45–47].

Mentions:
At the functional level, AD has been proposed to be characterized by early synaptic abnormalities [37]. AD mouse models display changes in synaptic structure, function and plasticity at a young age, [38,39]. It has also been hypothesized that synaptic dysfunction is a major cause of the cognitive deficits observed in AD [27,29,40]. Because GW3965 improved cognitions in an hippocampus-dependent paradigm such as MWM, we tested whether the drug had any effect on hippocampus synaptic transmission. Acute application of 0.5 μM GW3965, produced a gradual potentiation of single fEPSPs evoked from Schaeffer collateral to CA1 neurons every 30 sec (Fig 9A), which was prevented by the protein synthesis inhibitors anisomycin or cyclohexemide [41] (Fig 9B and 9C). Interestingly 0.1 μM GW3965 did not significantly affected fEPSP amplitude or slope (Fig 9B and 9C), while 1 μM was synaptotoxic (not shown). As it has been reported before [42] also in a dose dependent manner, bath application of oligomeric Aβ42 (oAβ42) peptides at 200 nM concentration in hippocampal brain slices of WT mice inhibited the induction of long term potentiation of the Schaeffer collateral-CA1 synapses, a proposed electrophysiological correlate to learning and memory (Fig 9D). No significant effect on LTP induction or maintenance was observed with 100 nM (oAβ42) peptides or scrambled control Aβ peptides (Fig 9D). Since for the LTP paradigm it is necessary to obtain a baseline fEPSP for 15 min (see methods for details) we used 0.1 μM GW3965 co-incubation with oAβ42, as shown in (Fig 9E and 9G), GW3965 at this concentration rescued LTP function. These experiments suggest that GW3965 at 0.1 μM independently of the acute effect on fEPSP amplitude or slope can rescue LTP. We also rule out that the slice incubation with 200 nM oAβ42 or 0.1 μM GW3965+ 200 nM oAβ42 had any effect on basal fEPSP amplitude or slope for the time of the recording (S4 File). We then asked whether the effect of GW3965 on LTP modulation by oAβ42 required protein synthesis. Since LTP induction has been shown to be protein synthesis dependent [43,44], anisomycin was used to block protein synthesis 30 min after LTP was induced, as shown in (Fig 9F). The effect of GW was not affected by this treatment. These experiments are consistent with the observations made by other groups, that LTP induction is protein synthesis dependent, but not its maintenance (for the time we studied here). The experiments also suggest that once the effect of GW3965 is established (during the incubation period) then it is independent of protein synthesis. This does not imply that protein synthesis is not required during its initial effect. [45–47].

Bottom Line:
In the GW3965 treated 3xTg-AD mice we also observed reduction in astrogliosis and increased number of stem and proliferating cells in the subgranular zone of the dentate gyrus.The effect of GW3965 on synaptic function was protein synthesis dependent.Our findings identify alternative functional/molecular mechanisms by which LXR agonists may exert their potential benefits as a therapeutic strategy against AD.

ABSTRACTAlzheimer's disease (AD) is the major cause of dementia worldwide. The pharmacological activation of nuclear receptors (Liver X receptors: LXRs or Retinoid X receptors: RXR) has been shown to induce overexpression of the ATP-Binding Cassette A1 (ABCA1) and Apolipoprotein E (ApoE), changes that are associated with improvement in cognition and reduction of amyloid beta pathology in amyloidogenic AD mouse models (i.e. APP, PS1: 2tg-AD). Here we investigated whether treatment with a specific LXR agonist has a measurable impact on the cognitive impairment in an amyloid and Tau AD mouse model (3xTg-AD: 12-months-old; three months treatment). The data suggests that the LXR agonist GW3965 is associated with increased expression of ApoE and ABCA1 in the hippocampus and cerebral cortex without a detectable reduction of the amyloid load. We also report that most cells overexpressing ApoE (86±12%) are neurons localized in the granular cell layer of the hippocampus and entorhinal cortex. In the GW3965 treated 3xTg-AD mice we also observed reduction in astrogliosis and increased number of stem and proliferating cells in the subgranular zone of the dentate gyrus. Additionally, we show that GW3965 rescued hippocampus long term synaptic plasticity, which had been disrupted by oligomeric amyloid beta peptides. The effect of GW3965 on synaptic function was protein synthesis dependent. Our findings identify alternative functional/molecular mechanisms by which LXR agonists may exert their potential benefits as a therapeutic strategy against AD.